Block Editor: Introduce block templates for classic themes.

With this patch, users will be able to create custom block based templates
and assign them to specific pages/posts.

Themes can also opt-out of this feature

Props bernhard-reiter, carlomanf.
Fixes #53176.


git-svn-id: https://develop.svn.wordpress.org/trunk@51003 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template.php

@@ -0,0 +1,228 @@
+<?php
+/**
+ * Block template loader functions.
+ *
+ * @package WordPress
+ */
+
+/**
+ * Find a block template with equal or higher specificity than a given PHP template file.
+ *
+ * Internally, this communicates the block content that needs to be used by the template canvas through a global variable.
+ *
+ * @since 5.8.0
+ *
+ * @param string $template  Path to the template. See locate_template().
+ * @param string $type      Sanitized filename without extension.
+ * @param array  $templates A list of template candidates, in descending order of priority.
+ * @return string The path to the Full Site Editing template canvas file, or the fallback PHP template.
+ */
+function locate_block_template( $template, $type, array $templates ) {
+	global $_wp_current_template_content;
+
+	if ( $template ) {
+		// locate_template() has found a PHP template at the path specified by $template.
+		// That means that we have a fallback candidate if we cannot find a block template
+		// with higher specificity.
+		// Thus, before looking for matching block themes, we shorten our list of candidate
+		// templates accordingly.
+
+		// Locate the index of $template (without the theme directory path) in $templates.
+		$relative_template_path = str_replace(
+			array( get_stylesheet_directory() . '/', get_template_directory() . '/' ),
+			'',
+			$template
+		);
+		$index                  = array_search( $relative_template_path, $templates, true );
+
+		// If the template hiearchy algorithm has successfully located a PHP template file,
+		// we will only consider block templates with higher or equal specificity.
+		$templates = array_slice( $templates, 0, $index + 1 );
+	}
+
+	$block_template = resolve_block_template( $type, $templates );
+
+	if ( $block_template ) {
+		if ( empty( $block_template->content ) && is_user_logged_in() ) {
+			$_wp_current_template_content =
+			sprintf(
+				/* translators: %s: Template title */
+				__( 'Empty template: %s' ),
+				$block_template->title
+			);
+		} elseif ( ! empty( $block_template->content ) ) {
+			$_wp_current_template_content = $block_template->content;
+		}
+		if ( isset( $_GET['_wp-find-template'] ) ) {
+			wp_send_json_success( $block_template );
+		}
+	} else {
+		if ( $template ) {
+			return $template;
+		}
+
+		if ( 'index' === $type ) {
+			if ( isset( $_GET['_wp-find-template'] ) ) {
+				wp_send_json_error( array( 'message' => __( 'No matching template found.' ) ) );
+			}
+		} else {
+			return ''; // So that the template loader keeps looking for templates.
+		}
+	}
+
+	// Add hooks for template canvas.
+	// Add viewport meta tag.
+	add_action( 'wp_head', '_block_template_viewport_meta_tag', 0 );
+
+	// Render title tag with content, regardless of whether theme has title-tag support.
+	remove_action( 'wp_head', '_wp_render_title_tag', 1 );    // Remove conditional title tag rendering...
+	add_action( 'wp_head', '_block_template_render_title_tag', 1 ); // ...and make it unconditional.
+
+	// This file will be included instead of the theme's template file.
+	return ABSPATH . WPINC . '/template-canvas.php';
+}
+
+/**
+ * Return the correct 'wp_template' to render for the request template type.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * Accepts an optional $template_hierarchy argument as a hint.
+ *
+ * @param string   $template_type      The current template type.
+ * @param string[] $template_hierarchy (optional) The current template hierarchy, ordered by priority.
+ * @return WP_Block_Template|null template A template object, or null if none could be found.
+ */
+function resolve_block_template( $template_type, $template_hierarchy ) {
+	if ( ! $template_type ) {
+		return null;
+	}
+
+	if ( empty( $template_hierarchy ) ) {
+		$template_hierarchy = array( $template_type );
+	}
+
+	$slugs = array_map(
+		'_strip_template_file_suffix',
+		$template_hierarchy
+	);
+
+	// Find all potential templates 'wp_template' post matching the hierarchy.
+	$query     = array(
+		'theme'    => wp_get_theme()->get_stylesheet(),
+		'slug__in' => $slugs,
+	);
+	$templates = get_block_templates( $query );
+
+	// Order these templates per slug priority.
+	// Build map of template slugs to their priority in the current hierarchy.
+	$slug_priorities = array_flip( $slugs );
+
+	usort(
+		$templates,
+		function ( $template_a, $template_b ) use ( $slug_priorities ) {
+			return $slug_priorities[ $template_a->slug ] - $slug_priorities[ $template_b->slug ];
+		}
+	);
+
+	return count( $templates ) ? $templates[0] : null;
+}
+
+/**
+ * Displays title tag with content, regardless of whether theme has title-tag support.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * @see _wp_render_title_tag()
+ */
+function _block_template_render_title_tag() {
+	echo '<title>' . wp_get_document_title() . '</title>' . "\n";
+}
+
+/**
+ * Returns the markup for the current template.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * @return string block tempate markup.
+ */
+function get_the_block_template_html() {
+	global $_wp_current_template_content;
+	global $wp_embed;
+
+	if ( ! $_wp_current_template_content ) {
+		if ( is_user_logged_in() ) {
+			return '<h1>' . esc_html__( 'No matching template found' ) . '</h1>';
+		}
+		return;
+	}
+
+	$content = $wp_embed->run_shortcode( $_wp_current_template_content );
+	$content = $wp_embed->autoembed( $content );
+	$content = do_blocks( $content );
+	$content = wptexturize( $content );
+	if ( function_exists( 'wp_filter_content_tags' ) ) {
+		$content = wp_filter_content_tags( $content );
+	} else {
+		$content = wp_make_content_images_responsive( $content );
+	}
+	$content = str_replace( ']]>', ']]&gt;', $content );
+
+	// Wrap block template in .wp-site-blocks to allow for specific descendant styles
+	// (e.g. `.wp-site-blocks > *`).
+	return '<div class="wp-site-blocks">' . $content . '</div>';
+}
+
+/**
+ * Renders a 'viewport' meta tag.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * This is hooked into {@see 'wp_head'} to decouple its output from the default template canvas.
+ */
+function _block_template_viewport_meta_tag() {
+	echo '<meta name="viewport" content="width=device-width, initial-scale=1" />' . "\n";
+}
+
+/**
+ * Strips .php or .html suffix from template file names.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * @param string $template_file Template file name.
+ * @return string Template file name without extension.
+ */
+function _strip_template_file_suffix( $template_file ) {
+	return preg_replace( '/\.(php|html)$/', '', $template_file );
+}
+
+/**
+ * Removes post details from block context when rendering a block template.
+ *
+ * @access private
+ * @since 5.8.0
+ *
+ * @param array $context Default context.
+ *
+ * @return array Filtered context.
+ */
+function _block_template_render_without_post_block_context( $context ) {
+	/*
+	 * When loading a template directly and not through a page
+	 * that resolves it, the top-level post ID and type context get set to that
+	 * of the template. Templates are just the structure of a site, and
+	 * they should not be available as post context because blocks like Post
+	 * Content would recurse infinitely.
+	 */
+	if ( isset( $context['postType'] ) && 'wp_template' === $context['postType'] ) {
+		unset( $context['postId'] );
+		unset( $context['postType'] );
+	}
+
+	return $context;
+}